Polymeric material, such as ultra-high molecular weight polyethylene (UHMWPE), is used in load bearing applications. In humans, it can be used in total joint prostheses. Wear of the polyethylene components over years is known to compromise the longevity and performance of total joints in the long-term. Radiation cross-linking has been shown to reduce the wear rate of polyethylene and thus extend the longevity of total joint reconstructions. Radiation cross-linking also generates residual free radicals, which are known to cause oxidation and embrittlement in the long-term. Therefore, it is crucial to either eliminate or stabilize the free radicals so that deleterious oxidation is avoided or minimized. One method of free radical elimination through irradiation and melting were described by Merrill et al. (see U.S. Pat. No. 5,879,400). This is an acceptable and widely used method; however, such a melt history also reduces the crystallinity of the polyethylene and thus affects its mechanical and fatigue properties (see Oral et al., Biomaterials, 27:917-925 (2006)).
Other methods that avoids melting after irradiation is the one described, among other things, by Muratoglu and Spiegelberg (see U.S. application Ser. No. 10/757,551, filed Jan. 15, 2004; US 2004/0156879). These methods use an anti-oxidant, such as α-tocopherol, to stabilize the free radicals in irradiated polymeric material and prevent long-term oxidation. According to certain embodiments of these methods, α-tocopherol can be incorporated into polymeric material after irradiation through contact and diffusion.
α-Tocopherol can be used to lessen or eliminate reactivity of the residual free radicals in irradiated UHMWPE to prevent oxidation. The incorporation of α-tocopherol into irradiated UHMWPE can be achieved through either blending α-tocopherol with the UHMWPE powder prior to consolidation or diffusing the α-tocopherol into UHMWPE after consolidation of powder, both of which are taught in U.S. application Ser. No. 10/757,551. The latter also can be performed after the consolidated UHMWPE is irradiated. Since radiation cross-links the UHMWPE and thus increases its wear resistance, it can be beneficial to irradiate the consolidated UHMWPE in its virgin state without any α-tocopherol present. On the other hand, cross-linking has been shown to decrease certain mechanical properties and fatigue resistance of UHMWPE (see Oral et al., Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE, Biomaterials, 27 (2006) 917-925). Wear of UHMWPE in joint arthroplasty is a surface phenomenon whereas fatigue crack propagation resistance is largely a property of the bulk, other than the surface. Therefore, UHMWPE with high cross-linking on the surface and less cross-linking in the bulk can be beneficial as an alternate bearing in joint arthroplasty. Oral et al. (Characterization of irradiated blends of α-tocopherol and UHMWPE, Biomaterials, 26 (2005) 6657-6663) have shown that when present in UHMWPE, α-tocopherol reduces the efficiency of cross-linking of the polymer during irradiation. Spatial control of vitamin E concentration followed by irradiation can spatially control cross-linking as well. It can be desirable to add α-tocopherol after radiation cross-linking if high cross-linking is desired and that is possible by diffusing α-tocopherol into irradiated and consolidated UHMWPE. Diffusion and penetration depth in irradiated UHMWPE has been discussed. Muratoglu et al. (see U.S. application Ser. No. 10/757,551, filed Jan. 15, 2004; US 2004/0156879) described, among other things, high temperature doping and/or annealing steps to increase the depth of penetration of α-tocopherol into irradiated UHMWPE. Muratoglu et al. (see U.S. Provisional Application Ser. No. 60/709,795, filed Aug. 22, 2005) described annealing in supercritical carbon dioxide to increase depth of penetration of α-tocopherol into irradiated UHMWPE. UHMWPE medical implants can have a thickness of up to 30 mm and sometimes larger. Penetrating such large implants with α-tocopherol by diffusion can take a long time, however. Also, it is preferable in some embodiments to diffuse α-tocopherol into an irradiated UHMWPE preform and subsequently machine that preform to obtain the finished implant. The preform has to be larger than the implant and therefore the diffusion path for α-tocopherol is increased.
A similar problem is often observed with polyethylene components that are fabricated with an integral metal piece. Often the metal piece is porous to allow bone in-growth for the fixation of the implant. In others, the metal piece is not porous and may be used to increase the structural integrity of the polyethylene piece. Therefore in the presence of an integral metallic piece the diffusion of α-tocopherol will either be slowed down near the surface covered with the porous metals or inhibited near the surface covered by a non-porous metal plate or rod.
It can be beneficial to have α-tocopherol present throughout the polymeric article to stabilize all free radicals and prevent long-term oxidation induced mechanical property changes.
In order to eliminate free radicals, several further methods can be used such as melting (see Muratoglu et al. U.S. application Ser. No. 10/757,551), mechanical deformation and recovery (see Muratoglu et al., U.S. application Ser. No. 11/030,115) or high pressure crystallization (see Muratoglu et al. U.S. application Ser. No. 10/597,652).
In order to increase the strength of UHMWPE, high pressure crystallization (HPC) of UHMWPE has been proposed. (See Bistolfi et al., Transactions of the Orthopaedic Research Society, 2005. 240; Oral et al., Transactions of the Orthopaedic Research Society, 2005. p. 988.; Muratoglu et al. U.S. Provisional Application No. 60/541,073, filed Feb. 3, 2004; and PCT/US2005/003305, filed Feb. 3, 2005). High pressure crystallization of unirradiated GUR1050 UHMWPE at above 160° C. and 300 MPa yielded an approximately 70% crystalline UHMWPE, compared to 50-60% for conventional UHMWPE. This is due to a phase transition of the UHMWPE crystals from the orthorhombic to the hexagonal phase at high temperatures and pressures as discussed above. In the hexagonal phase crystals grow to larger sizes and crystallinity increases (see Bassett et al., J Appl. Phys., 1974, 45(10): p. 4146-4150).
It can be advantageous to have α-tocopherol present throughout all or part of the polymeric article in order to stabilize all free radicals and prevent long-term oxidation induced mechanical property changes. It also can be advantageous to have a medical implant, or any polymeric component thereof, doped with a spatial control of antioxidant distribution. This spatial control can be achieved by having gradual changes or step changes in the concentration of antioxidant. It also can be advantageous to have a medical implant with a spatial control of cross-linking. For example, Muratoglu et al. (see U.S. application Ser. No. 10/433,987, filed on Dec. 11, 2001) describe a UHMWPE with gradient cross-linking perpendicular to the irradiation direction by shielding.
This application describes UHMWPE medical implants that have a spatial control of cross-linking due to irradiation of UHMWPE containing a spatially controlled distribution of antioxidant.
High concentrations of antioxidants, for example, α-tocopherol, near the surface of a polymeric material can lead to elution of α-tocopherol into the joint space after implantation. α-tocopherol can elute out of the implants over time, especially at the human joint temperature of about 37.5° C. to 40° C. When stored in air or in water at 40° C., the irradiated and α-tocopherol-doped UHMWPE loses about 10% of the α-tocopherol over about the first six months. The presence of excess α-tocopherol in the joint space may possibly lead to an adverse biological response. In order to avoid such complication, α-tocopherol can be extracted from the polymeric material prior to placement and/or implantation into the body. In order to minimize the elution of α-tocopherol in vivo, a suitable method is necessary to extract the α-tocopherol from the surface regions of an α-tocopherol-containing cross-linked oxidation-resistant polymeric material. However, such achievement were not possible until the present invention.